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The initial nucleophile attacks silicon, not the nitrile or an alkene carbon. Each trimethylsilyl group has an electrophilic Si center. A methyl group from methyllithium attacks Si, passing through a pentacoordinate silicon arrangement; Si–O cleavage releases $\mathrm{SiM}{\mathrm{e}}_4$ and the lithium salt of the oxygen-containing fragment. Cleaving the silyl enol ether generates a lithium enolate; cleaving the aryl silyl ether generates a phenoxide. Neither operation needs a new deprotonation at a carbon stereocenter.Here $R-O^{-}$ represents either the phenoxide or the oxygen-centered enolate contributor. Excess reagent permits both cleavages. The pre-existing methyl/nitrile stereocenter retains its configuration: it has no hydrogen to remove, and neither bond attached to that center is broken.

After HMPA is added, each anionic oxygen attacks the carbonyl carbon of allyl chloroformate. The C=O pi electrons move to the carbonyl oxygen, giving a tetrahedral intermediate; collapse expels chloride. The enolate becomes an allyl enol carbonate, and the phenoxide becomes an aryl allyl carbonate, each containing $\mathrm{O}-\mathrm{C}(\mathrm{O})-\mathrm{OC}{\mathrm{H}}_2\mathrm{CH}=\mathrm{C}{\mathrm{H}}_2$. oxygen acylation accounts for preservation of the enol double bond and for the two newly installed carbonate groups. carbon attack would give a different carbon skeleton. The two oxygens are trapped because they were selectively unmasked by desilylation, rather than because every ether oxygen in the molecule is equally reactive.

Lithium diisopropylamide removes an alpha proton from the ethyl side of the ketone. The C–H electrons form the C=C part of a lithium enolate, while the C=O pi electrons move to oxygen. The phenyl side has no competing alpha C–H site. Diisopropylamine is the conjugate-acid byproduct.The carbon end of this enolate attacks the primary carbon bearing iodine in $\mathrm{IC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{OMe}$. The C–I bond breaks to iodide in an SN2 reaction, and carbonyl reformation givesThus a three-carbon methoxypropyl chain is attached at the ketone alpha carbon. The remote methyl ether survives. The hindered amide base is chosen to generate the enolate rather than to add to the carbonyl. No absolute stereochemical preference is specified for the new stereocenter under these achiral conditions.

An enolate is an ambident nucleophile: the negative charge is concentrated toward oxygen, while its occupied conjugated orbital also has substantial reactive amplitude at the alpha carbon. An alkyl iodide presents an accessible, polarizable carbon electrophile. carbon attack through an SN2 reaction is generally kinetically favorable, while the more strongly charged oxygen is often coordinated to the counterion or solvated. The carbonyl reforms as the new C–C bond is made.carbon alkylation restores the carbonyl and makes the new C–C bond. oxygen alkylation instead gives an enol ether. The two outcomes are not resonance contributors of one product, and their ratio is not determined just by where the largest formal negative charge is drawn. electrophile, counterion, solvent and aggregation can change selectivity; hard silicon electrophiles and suitable acylating reagents often favor oxygen trapping. Product stability alone would not explain an irreversible kinetic choice.

HMPA coordinates Li${}^{+}$ through its phosphoryl oxygen and makes the anionic oxygen sites more available for acylation. Strong cation solvation changes contact ion pairing, chelation and aggregate structure. It can therefore accelerate trapping of both the enolate and phenoxide by the chloroformate, helping realize the observed O-acylation pathway.

It is too strong to assert that HMPA necessarily produces completely free, monomeric anions. In a primary study of a lithium enolate in THF, low-temperature multinuclear NMR and reaction kinetics identified an HMPA-coordinated dimer as the reactive form, even with excess HMPA.^[ref] The useful conclusion here is altered lithium coordination and enhanced oxygen accessibility; the exact aggregate of this bifunctional substrate is not supplied.

Hydroxide initially deprotonates a little thiophenol, generating the reactive thiolate $\mathrm{Ph}{\mathrm{S}}^{-}$. Number the extended conjugated system $\mathrm{O}=C1-C2=C3-C4=C5$, where C1–C2–C3 is the ring enone and C4=C5 is the attached vinyl group. sulfur attacks the terminal delta carbon C5. The pi bonds relay toward oxygen, generating an extended enolate.

proton transfer at C4, either directly from thiophenol or through solvent-mediated enolization, gives the product retaining the conjugated ring enone. The product is 3-[2-(phenylsulfanyl)ethyl]cyclohex-2-en-1-one, drawn below. Its side chain is $-\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{SPh}$; the ring C2=C3 double bond remains. This is net sulfur/hydrogen addition to the terminal part of the extended system, proceeding through 1,6-Conjugate addition and protonation of the extended enolate.

The terminal site is less crowded than the ring beta carbon, and the isolated product retains favorable carbonyl–alkene conjugation. Reversible proton transfers and conjugate addition under thiolate catalysis can favor that product over an unconjugated enone-derived alternative. Transfer of a proton from another PhSH molecule regenerates PhS${}^{-}$, explaining why a catalytic quantity of hydroxide suffices. The net balance is one thiophenol added, ${\mathrm{C}}_8{\mathrm{H}}_{10}\mathrm{O}+{\mathrm{C}}_6{\mathrm{H}}_6\mathrm{S}={\mathrm{C}}_{14}{\mathrm{H}}_{16}\mathrm{OS}$; carbonyl 1,2 addition would instead produce a different functional group and is not favored for this soft sulfur nucleophile.

Hydride removes the remaining alpha proton at the substituted benzylic carbon, releasing hydrogen and forming the conjugated enolate. Its oxygen attacks the carbonyl carbon of allyl chloroformate. Collapse of the tetrahedral intermediate expels chloride and gives the allyl enol carbonate, with the double bond between the original carbonyl carbon and the cyanomethyl-bearing carbon.

This is O-acylation of an enolate: the carbonyl oxygen is trapped as $\mathrm{OC}{\mathrm{O}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{CH}=\mathrm{C}{\mathrm{H}}_2$, retaining the enol double bond. The competing carbon-centred resonance representation explains possible C-acylation in other circumstances, but it would give a different connectivity from E. The indicated product selects the oxygen-trapping pathway and the conjugated enolate regioisomer.

At pH 7, a glutamate residue is normally carboxylate. Its most natural role is general-base activation of the attacking water. Draw the carboxylate oxygen lone pair toward a water H, the water O–H pair toward its oxygen, and that oxygen's lone pair toward C3 while the conjugated pi system moves toward the carbonyl oxygen:This can be concerted, avoiding a freely diffusing hydroxide intermediate. The protonated glutamate subsequently returns its proton to the enolate system, directly or through solvent, regenerating carboxylate. A favorable arrangement is needed; ordinary carboxylate does not quantitatively deprotonate bulk water in a prior equilibrium.

The side-chain amide of glutamine is neutral. Its N–H can donate a hydrogen bond to the carbonyl oxygen and stabilize developing enolate charge, while its amide oxygen can accept a hydrogen bond to orient attacking water. Draw $\mathrm{Gln}-\mathrm{CON}{\mathrm{H}}_2\cdots \mathrm{O}=C1-C2=C3$, with the dotted interaction from amide N–H to substrate oxygen. This assists polarization and transition state stabilization; the amide nitrogen is not an ordinary amine base because its lone pair is delocalized into its own carbonyl.

The guanidinium group of arginine is positively charged. It can stabilize developing negative charge by hydrogen bonding and electrostatic attraction, drawn $\mathrm{Arg}-\mathrm{guanidiniu}{\mathrm{m}}^{+}\cdots {\mathrm{O}}^{\delta -}-C1$. Its delocalized cation is not the preferred neutral general base at pH 7. Productive catalysis requires stronger stabilization of the reacting arrangement than of the unreactive ground-state complex.

Phenylalanine and leucine have hydrocarbon side chains and provide no conventional acid/base or hydrogen-bond donor/acceptor group here. They can position the enone, exclude unproductive solvent and select the attacked face through hydrophobic effect and steric hindrance. Those binding effects can assist catalysis but should not be drawn as an imaginary side-chain proton relay. The original schematic depicts the Glu water relay and the Gln/Arg oxyanion-stabilizing contacts.

Use C1 for the carbonyl carbon, C2 for the adjacent carbon, and C3 for the beta carbon of the reactive enone fragment $\mathrm{R}-C1(=\mathrm{O})-C2H=C3H-{\mathrm{R}}^{\prime }$. water adds conjugately at C3, giving a beta-hydroxy ketone. The oxygen lone pair of neutral water attacks C3; move the C2=C3 pi pair toward C1–C2 and the carbonyl pi pair onto oxygen. This produces the charge-separated enolate–oxonium intermediateA second water molecule can relay a proton from the newly attached oxonium oxygen to the enolate system. Using the carbon-centered resonance structure, reform C1=O and protonate C2 while the donating O–H bond returns its pair to the added oxygen. Equivalently, oxygen protonation first gives an enol, followed by water-mediated keto-enol tautomerism. Either path yields a neutral beta-hydroxy ketone and regenerates the relay water. There is no need to introduce externally supplied H${}^{+}$ or OH${}^{-}$; solvent molecules perform the proton transfers. The uncatalyzed charge separation has a large barrier, so this mechanism describes a slow formal pathway, not a claim of rapid hydration in pure water.

Hydride is delivered to steroid C5 and a proton to C4; the C3 ketone is retained. Use the conjugated fragment $\mathrm{O}=C3-C4=C5$. Carbonyl hydrogen bonds from Tyr91–OH and protonated Glu57 lower the barrier to conjugate reduction. Draw a curved arrow from the nicotinamide C4–H bond of NADPH to steroid C5, shift the C4=C5 pi pair toward C3–C4, and shift C3=O pi electrons onto oxygen. The nicotinamide ring rearomatizes to NADP+; the steroid becomes a bound enolate, or its protonated enol equivalent. electron shifts across the nicotinamide ring must accompany C–H bond cleavage rather than leaving an unexplained neutral carbon radical.

Next, reform C3=O and protonate steroid C4 from an appropriately positioned acid, directly or through an enol/proton-relay route. This regenerates the catalytic acid after solvent reprotonation. The enzyme fixes the hydride-accepting face so that the new C5–H has the 5α configuration, giving dihydrotestosterone; delivery to the opposite face would give a different ring-junction configuration. This is not simple 1,2 reduction of the C3 carbonyl to an alcohol. The net local bookkeeping is one hydride plus one proton across C4=C5, with two electrons supplied by NADPH. The original reactive-fragment diagram shows the enolate-forming arrows and the separate protonation step.

finasteride is reduced at C1, leaving a C2 enolate that attacks nicotinamide C4 to make the covalent C–C adduct. In the inhibitor's unsaturated lactam, the reactive chain is $\mathrm{O}=C3-C2=C1$, with N at steroid position 4. Activate the C3 carbonyl by hydrogen bonding; draw the reduced cofactor's C4–H bond toward inhibitor C1, the C1=C2 pi pair toward C2–C3, and the C3=O pi pair toward oxygen. This gives NADP+ next to the C2-centered resonance form of the inhibitor enolate.

Instead of completing reduction by protonating C2, move the C2 lone pair toward the electrophilic C4 of the oxidized nicotinamide ring. Shift the pyridinium pi system toward its positively charged N, and reform the inhibitor carbonyl if using the oxygen-centered enolate representation. The result is the C2(inhibitor)–C4(nicotinamide) bond and a neutral dihydronicotinamide ring. Numbering the two molecules separately prevents confusing cofactor C4 with the inhibitor's N4. The lactam N–H remains present and can help position the inhibitor through hydrogen bonding; it does not form a covalent bond to the protein.

This is mechanism-based inhibition: enzyme chemistry generates the species that traps its own oxidized cofactor. Isolation of a cofactor–inhibitor adduct after protein denaturation, rather than persistent attachment to a denatured protein, distinguishes it from covalent modification of an amino acid side chain. The original mechanistic study combined radiolabel recovery, purification and spectroscopy to establish the adduct; its slow breakdown also cautions against treating “irreversible” as an absolute statement for every timescale.^[ref]

The most acidic sites are: acetic-acid O–H, $\mathrm{p}{K}_a\approx 4.8$; phenol O–H, $\approx 10$; ethanal alpha-CH${}_3$, $\approx 17$–$20$; propyne terminal ≡C–H, $\approx 25$. These are ordinary approximate organic-chemistry estimates; exact acidities depend on solvent and reference convention.

Acetic acid gives a carboxylate in which negative charge is delocalized over two oxygens through equivalent resonance structures. Phenol gives a phenoxide with delocalization into the aromatic system, but its important contributors are less equivalent and do not stabilize the charge as effectively as a carboxylate.

For ethanal, removal of a methyl hydrogen gives the enolate ${}^{-}\mathrm{C}{\mathrm{H}}_2-\mathrm{CH}=\mathrm{O}\leftrightarrow \mathrm{C}{\mathrm{H}}_2=\mathrm{CH}-{\mathrm{O}}^{-}$. The aldehydic hydrogen is not the preferred acidic site: removing it does not give this alpha-carbon/oxygen delocalization. For propyne, loss of the terminal hydrogen gives an acetylide with a lone pair in an $sp$ orbital. Its greater s character stabilizes negative charge relative to an ordinary $s{p}^2$ or $s{p}^3$ carbanion; the methyl hydrogens are much less acidic.

Disconnect the N–cyclohexyl bond as a late reductive amination of a primary amine with cyclohexanone. Disconnect the exocyclic methylene as a Wittig methylenation of a substituted cyclohexanone. The useful ring precursor is 3-methyl-5-substituted cyclohex-2-en-1-one, accessible by a Robinson annulation. The pendant substituent throughout ring construction should be $(\mathrm{C}{\mathrm{H}}_2{)}_4\mathrm{NHBoc}$.

Start with nucleophilic substitution of the supplied bromo ester by cyanide:Reduce the nitrile selectively to the primary amine while retaining the ester, for example by controlled catalytic hydrogenation with Raney nickel in the presence of ammonia, and trap the amine promptly as its Boc carbamate. Avoid heating or concentrating the free amino ester, which can cyclize to a lactam. lithium aluminium hydride would be unsuitable at this stage because it also reduces the ester.

Partial reduction of the protected ester with DIBAL at low temperature gives $\mathrm{BocNH}(\mathrm{C}{\mathrm{H}}_2{)}_4\mathrm{CHO}$. The carbon of the original nitrile is now the methylene nearest nitrogen; the original ester carbonyl is the aldehyde carbon. Condense this aldehyde with acetone and dehydrate to $\mathrm{BocNH}(\mathrm{C}{\mathrm{H}}_2{)}_4\mathrm{CH}=\mathrm{CHCOMe}$.

Now add the enolate of a second acetone molecule in a Michael addition. With $R=(\mathrm{C}{\mathrm{H}}_2{)}_4\mathrm{NHBoc}$, the resulting chain isIntramolecular aldol condensation gives 3-Me-5-R-cyclohex-2-en-1-one. The beta methyl derives from the carbonyl end attacked in the ring closure; the pendant aminoalkyl chain remains at C5. Hydrogenate the ring C=C selectively, retaining the ketone, then methylenate the ketone with preformed $\mathrm{P}{\mathrm{h}}_3\mathrm{P}=\mathrm{C}{\mathrm{H}}_2$. Do the hydrogenation before installing the exocyclic methylene so that it is not hydrogenated away.

Remove Boc under brief mild conditions, checking for acid-induced exocyclic alkene isomerization. reductive amination of the resulting primary amine with cyclohexanone, followed by selective imine reduction, installs the secondary N-cyclohexyl group. Use controlled stoichiometry to suppress tertiary amine formation. The methyl and four-carbon aminoalkyl substituents occupy the required 1,3-related ring positions, with the exocyclic methylene at the former carbonyl. No ring stereochemistry is specified. The target contains eighteen carbons: six ring carbons, one ring methyl, one methylene, four in the linker, and six in the N-cyclohexyl group.

Disconnect the tertiary alcohol at its bond to the alkenyl group. The required carbonyl partner is $\mathrm{MeCOCH}(\mathrm{Me})\mathrm{Et}$; the other partner is an E-configured $\mathrm{PhC}{\mathrm{H}}_2\mathrm{CH}=\mathrm{CH}$ organometallic reagent. This disconnection fixes the alkene geometry before the alcohol is made.

First form the ethyl-side enolate of butan-2-one under equilibrating conditions, then trap it with ethyl iodide and isolate the required monoalkylated ketone:The incoming group is Et: methylation would give a carbon skeleton one carbon too short. Because an unsymmetrical ketone can form two enolates and undergo repeated alkylation, regioisomer separation and controlled conversion are part of this step.

Independently perform a halo-Wittig reaction on phenylacetaldehyde using $\mathrm{P}{\mathrm{h}}_3\mathrm{P}=\mathrm{CHBr}$ and isolate E-$\mathrm{PhC}{\mathrm{H}}_2\mathrm{CH}=\mathrm{CHBr}$ from any geometrical mixture. magnesium insertion in dry ether gives the corresponding vinylmagnesium halide; add the isolated ketone at low temperature and protonate the resulting alkoxide. Addition to C=O preserves the pre-existing C=C geometry:The E alkene and the complete fifteen-carbon skeleton are obtained. No absolute configuration is specified at the newly formed alcohol centre, so an unselected stereoisomeric mixture there is acceptable. An alternative carbon–carbon disconnection is acetylide addition to the same ketone followed by a controlled trans-selective alkyne reduction; the vinyl-reagent route avoids relying on reduction of a tertiary propargylic alcohol.

Open the lactone formally at its acyl–oxygen bond, then replace the resulting secondary alcohol by its ketone precursor. With $R=\mathrm{CH}(\mathrm{Me})\mathrm{CH}(\mathrm{Me}{)}_2$, the useful sequence of retrosynthetic intermediates isDisconnect the last compound as a Michael addition product of the methyl-side enolate of $\mathrm{RCOC}{\mathrm{H}}_3$ and ethyl methacrylate:The substituted carbon of the methacrylate becomes the methyl-bearing carbon alpha to the lactone carbonyl. The ketone carbon becomes the oxygen-bearing ring carbon. This checks the six-membered ring connectivity.

The required branched methyl ketone itself comes from butan-2-one by ethyl-side aldol addition to acetone, followed by dehydration and selective hydrogenation:Choose and isolate the aldol regioisomer shown. For the subsequent Michael step, use a bulky base at low temperature to favour its less hindered methyl-side kinetic enolate. Reduce the resulting ketone selectively with $\mathrm{NaB}{\mathrm{H}}_4$ while retaining the ester, hydrolyse the ester, then acidify and promote intramolecular esterification with water removal. The hydroxy acid closes to the required six-membered lactone. There is no specified stereochemical relationship to enforce, but a stereochemically pure target would require an additional selectivity or resolution step.

There are three largely independent constructions: a substituted Robinson-annulation core, a protected vicinal diol, and the ester acyl fragment. Disconnect the ester first to an allylic alcohol on the bicyclic ring and the acid $\mathrm{H}{\mathrm{O}}_2\mathrm{CCH}(\mathrm{Et})\mathrm{CH}(\mathrm{Me}{)}_2$. Disconnect the benzylidene acetal to a side chain $-\mathrm{C}{\mathrm{H}}_2\mathrm{CH}(\mathrm{OH})\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ and benzaldehyde. Reversing 1,2-reduction of the allylic alcohol gives the corresponding bicyclic enone; its six-membered enone ring is a Robinson disconnection back to a suitably alpha-substituted cyclohexanone and methyl vinyl ketone.

An efficient way to protect the side chain before annulation is to prepare 4-(bromomethyl)-2-phenyl-1,3-dioxolane from $\mathrm{BrC}{\mathrm{H}}_2\mathrm{CH}(\mathrm{OH})\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ and benzaldehyde with mild acid and water removal. The desired five-membered acetal protects the adjacent hydroxyl groups, while its external primary bromide remains an alkylating site. Alpha alkylation of the indicated cyclohexanone with this bromide gives a ring bearing $-\mathrm{C}{\mathrm{H}}_2\mathrm{CH}(\mathrm{O}-)\mathrm{C}{\mathrm{H}}_2\mathrm{O}-$ in the benzylidene-protected form. There is no need to expose the eventual ring enone to a nonselective late dihydroxylation.

Generate the enolate at the other, less substituted alpha position of the cyclohexanone and add methyl vinyl ketone. Hydrolysis of an enamine-mediated Michael variant or direct enolate trapping gives the appropriate 1,5-dicarbonyl. Intramolecular aldol condensation closes a six-membered ring onto the original cyclohexanone carbonyl. Dehydration gives the fused bicyclic enone, with the protected three-carbon side chain on the adjacent carbon of the original ring. Regioselecting the initial Michael addition is essential: reaction at the side-chain-bearing alpha site would place that substituent at the wrong fusion position.

Reduce the new enone carbonyl by 1,2-addition, for example sodium borohydride with cerium(III) chloride under Luche-type conditions, to retain the displayed ring C=C bond and produce the allylic alcohol. The benzylidene acetal should be maintained through these neutral/basic operations. The target has no wedge/dash assignments, so a constitutional solution does not require a claim of a unique fused-ring/alcohol diastereomer; isolate the required constitutional annulation product.

For the acyl branch, form the enolate of the indicated methyl butanoate, add acetone, then dehydrate the beta-hydroxy ester:Hydrogenate this isolated alkene before coupling it to the bicyclic substrate, giving $\mathrm{Me}{\mathrm{O}}_2\mathrm{CCH}(\mathrm{Et})\mathrm{CHM}{\mathrm{e}}_2$. saponification gives $\mathrm{H}{\mathrm{O}}_2\mathrm{CCH}(\mathrm{Et})\mathrm{CHM}{\mathrm{e}}_2$; activate it as its acid chloride or use a mild ester-coupling reagent, then acylate the bicyclic allylic alcohol. Performing the hydrogenation on the separate acyl branch avoids reducing the target ring alkene. Strong acid should be avoided during the final esterification because the benzylidene group is acid-labile.

The carbon audit is particularly useful: the fused rings contain six carbons from cyclohexanone plus four from methyl vinyl ketone; the side chain has three carbons from the protected bromodiol; the benzylidene unit supplies its acetal carbon and phenyl group. In the acyl fragment, the four butanoate acyl carbons plus three acetone carbons give seven; the starting ester methoxy carbon is lost during hydrolysis.

Both side chains have the order ring–$\mathrm{C}{\mathrm{H}}_2\mathrm{CH}=\mathrm{CHMe}$, and both printed double bonds have the Z geometry. The two carbon side chains are cis to one another; equivalently the labelled ring H and ring Me are cis on the opposite face. A useful common late precursor therefore has two cis ring–$\mathrm{C}{\mathrm{H}}_2\mathrm{CHO}$ substituents, which can undergo two Z-selective ethylidene Wittig reactions.

Use the starting 2-methylcyclohex-2-en-1-one in a reductive alpha-alkylation. Conjugate reduction under dissolving-metal/enolate-trapping conditions, followed by a primary electrophile $\mathrm{BrC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{OTBS}$, installs a protected two-carbon alcohol side chain at the original methyl-bearing alpha carbon. This gives 2-methyl-2-(2-siloxyethyl)cyclohexanone. The carbon at that position is now quaternary, with its original methyl retained. Alternatively, reduce the enone to 2-methylcyclohexanone and deliberately form/select its more substituted enolate before alkylation; its opposite kinetic enolate would give the wrong regioisomer.

Convert the remaining ring ketone to an exocyclic enoate by Horner–Wadsworth–Emmons olefination with a phosphonoacetate:Hydrogenate this sole alkene to give ring–$\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{O}}_2\mathrm{Et}$ at that carbon. Select the diastereomer with this substituent cis to the pre-existing protected hydroxyethyl side chain, so the newly installed ring H is cis to ring Me. Less hindered-face hydrogenation can assist this relationship, but the route should include separation/verification of the desired cis product rather than assert complete stereoselectivity without data.

Reduce the ester with lithium aluminium hydride to ring–$\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ and remove the TBS group from the other side chain. Both appendages are now two-carbon primary alcohols. Oxidize them to aldehydes under non-overoxidizing conditions, for example Dess–Martin oxidation, obtaining the cis dialdehyde with two ring–$\mathrm{C}{\mathrm{H}}_2\mathrm{CHO}$ groups. Treat with excess salt-controlled, nonstabilized ethylidenetriphenylphosphorane under conditions favouring kinetic Z olefination; isolate the Z,Z product. Each aldehyde carbon becomes the inner vinylic carbon of its crotyl group, while the ylide supplies $\mathrm{CHMe}$.

This order postpones both stereodefined alkenes until after hydrogenation, so they are not accidentally saturated. It also avoids assuming that a crotyl grignard reagent adds without allylic transposition or that direct alcohol deoxygenation fixes the required ring stereochemistry. An achiral synthesis gives a racemic mixture pair with the specified relative configuration.

Use the nitro group as a masked carbonyl and a carbon nucleophile. The most informative disconnection reverses the six-membered aldol annulation. Before that closure, the cyclopentanone beta carbon bearsThe carbon carrying $\mathrm{N}{\mathrm{O}}_2$ will become the lower carbonyl of the final six-membered ring by a Nef reaction. It comes from the specified nitromethane. The remote methyl ketone supplies the upper methyl-substituted alkene carbon after aldol condensation.

First, use a mild base to generate a nitromethane-derived nitronate and add it conjugatively to the indicated cyclopent-2-en-1-one. Protonation gives 3-(nitromethyl)cyclopentanone. Deprotonate its nitromethyl group and perform a second Michael addition to methyl vinyl ketone, giving $\mathrm{cyclopentanone}-!C3-\mathrm{CH}(\mathrm{N}{\mathrm{O}}_2)\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{COC}{\mathrm{H}}_3$. The nitro-stabilized carbon is appreciably more acidic than a simple ketone alpha carbon, permitting selective nitronate formation; control the acceptor stoichiometry to avoid a further Michael alkylation of the remaining nitro-bearing H.

Form the enolate/enamine at the cyclopentanone carbon between its carbonyl and the side-chain-bearing carbon. Intramolecular attack on the remote methyl ketone closes a six-membered ring; the two shared atoms are the original enone alpha and beta carbons. Hydrolysis if required and dehydration create the alkene between the original alpha carbon and the newly formed tertiary carbinol carbon. That carbinol carbon retains its methyl substituent, placing the methyl exactly as required. The original cyclopentanone carbonyl remains the carbonyl of the five-membered ring.

Finally, form the secondary nitronate and carry out controlled acidic Nef hydrolysis to replace its $\mathrm{CH}(\mathrm{N}{\mathrm{O}}_2)$ unit by $\mathrm{C}=\mathrm{O}$. The resulting fused product contains the two required ketones. Carrying out the annulation before exposing this second ketone reduces competing enolate/aldol pathways that would be available in the corresponding triketone. The nitro intermediate's competing deprotonation means that the aldol step needs conditions designed to access the ring carbonyl enolate/enamine; simply treating every carbonyl position as equally reactive would not establish the route.

The carbon ledger is $5$ from cyclopentenone, $1$ from nitromethane and $4$ from methyl vinyl ketone, giving the nine-carbon fused skeleton plus its methyl substituent. A primary-nitro Nef reaction performed before the second carbon alkylation would instead give an aldehyde and would lose this useful protected-carbonyl construction.

Let $R=\mathrm{CH}(\mathrm{Me})\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_3$. The useful disconnections are benzoate formation, carbonyl reduction and thiol conjugate addition:The sulfur-bearing carbon is the enone beta carbon, so this is a Michael disconnection, not attack of thiophenol on the alcohol-derived central carbon. Reduce the saturated ketone with sodium borohydride after installing SPh, then benzoylate the alcohol with benzoyl chloride and pyridine or a tertiary amine.

Disconnect the enone to $\mathrm{RCOC}{\mathrm{H}}_3$ and formaldehyde. Kinetic deprotonation on the methyl side of the hindered unsymmetrical ketone, trapping with formaldehyde, and dehydration gives $\mathrm{RCOCH}=\mathrm{C}{\mathrm{H}}_2$. This order leaves the substituted $R$ side unchanged. An alpha-methylenation/Mannich-base-elimination variant is an equivalent way of installing the terminal enone.

Finally, $\mathrm{RCOC}{\mathrm{H}}_3=\mathrm{C}{\mathrm{H}}_3\mathrm{COCH}(\mathrm{Me})\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_3$ disconnects by alpha alkylation to butan-2-one and 1-iodopropane. Use the more substituted, ethyl-side enolate of butan-2-one for this step, rather than its usual methyl-side kinetic enolate; an equilibrated/regioisomerically selected enolate or enol derivative is required, and the desired monoalkylated ketone should be isolated. Three carbons from propyl iodide and one from formaldehyde increase the original four-carbon ketone to the required eight-carbon alcohol/thioether skeleton. Choosing ethyl iodide here would leave the target short by one carbon.

This is a constitutional synthesis: the target drawing specifies no configuration at either newly formed stereogenic site, so no unproved stereoselective reduction or alkylation is needed. Regioisomeric enolate alkylation and competing overalkylation are the practical selectivity issues to control, rather than something that follows from writing “base”.

The target is 5-Aminohexanoic acid, $\mathrm{H}{\mathrm{O}}_2\mathrm{C}(\mathrm{C}{\mathrm{H}}_2{)}_3\mathrm{CH}(\mathrm{N}{\mathrm{H}}_2)\mathrm{Me}$. Disconnect its terminal amine/carboxylic-acid relationship through hydrolysis of 6-methylpiperidin-2-one. This six-membered lactam follows from a regioselected Beckmann ring expansion of 2-methylcyclopentanone oxime.

Start with cyclopentanone. Form its enolate with lithium diisopropylamide in dry THF at low temperature, add methyl iodide, then quench to obtain 2-methylcyclopentanone. Control the methyl iodide/base stoichiometry to suppress a second alkylation. React the ketone with hydroxylamine to form its oxime and select the geometrical oxime isomer in which the bond to the methyl-substituted adjacent ring carbon is anti to the departing oxime oxygen group.

Activate the oxime, for example under appropriate acid-promoted Beckmann conditions. The anti methyl-bearing C–C bond migrates onto nitrogen as the N–O-derived leaving group departs, inserting nitrogen between the original carbonyl carbon and the methyl-substituted ring carbon. Hydration/tautomerization gives 6-methylpiperidin-2-one. The opposite oxime geometry would insert nitrogen on the other side and ultimately place methyl at a different position in the amino acid; oxime geometry is therefore an essential part of this proposal.

Hydrolyse the lactam with aqueous acid and heat, then adjust the work-up to the desired acid/ammonium form. Cleavage of its acyl–nitrogen bond opens the ring to $\mathrm{H}{\mathrm{O}}_2\mathrm{C}(\mathrm{C}{\mathrm{H}}_2{)}_3\mathrm{CH}(\mathrm{N}{\mathrm{H}}_2)\mathrm{Me}$ after suitable neutralization. All five original cyclopentanone carbons are retained, and the one methyl carbon comes from methyl iodide. A racemic mixture product is appropriate to the stereochemically unspecified target. The reaction sequence is an organic ring-expansion synthesis and requires no biological transformation.

Open the cyclic ester at its acyl–oxygen bond. The resulting delta-hydroxy acid isOxidation-state reversal at the benzylic alcohol gives $\mathrm{H}{\mathrm{O}}_2\mathrm{C}-\mathrm{CH}(\mathrm{nPr})-\mathrm{C}{\mathrm{H}}_2-\mathrm{CH}(\mathrm{iPr})-\mathrm{C}(=\mathrm{O})\mathrm{Ph}$. This identifies the key Michael disconnection between the central $\mathrm{C}{\mathrm{H}}_2$ and $\mathrm{CH}(\mathrm{iPr})$ groups:The nucleophile is the enolate or enamine of the phenyl ketone. The electrophile's propyl substituent must be alpha to its ester carbonyl; using a beta-propyl acrylate would move the substituent to the wrong ring position.

Disconnect $\mathrm{PhCOC}{\mathrm{H}}_2\mathrm{iPr}$ further to the indicated acetophenone. Crossed aldol addition to acetone followed by dehydration gives $\mathrm{PhCOCH}=\mathrm{CM}{\mathrm{e}}_2$; selective conjugate reduction or controlled alkene hydrogenation then gives $\mathrm{PhCOC}{\mathrm{H}}_2\mathrm{CHM}{\mathrm{e}}_2$. This avoids requiring an efficient SN2 alkylation with a secondary isopropyl halide. The alpha-propyl acrylate can be obtained by alpha-hydroxymethylation/dehydration of ethyl pentanoate, $\mathrm{Et}{\mathrm{O}}_2\mathrm{CC}{\mathrm{H}}_2\mathrm{nPr}$, using formaldehyde.

In the forward direction, perform the Michael coupling, reduce the phenyl ketone selectively with sodium borohydride while retaining the ordinary ester, hydrolyse the ester, and cyclize the delta-hydroxy acid by controlled acid-catalysed lactonization or an activated-acid coupling. The six-membered ring then has the order carbonyl–$\mathrm{CH}(\mathrm{nPr})$–$\mathrm{C}{\mathrm{H}}_2$–$\mathrm{CH}(\mathrm{iPr})$–$\mathrm{CH}(\mathrm{Ph})$–O. This atom order checks all three substituent positions. No relative configuration is specified, so the scheme does not imply a single diastereomer from the Michael addition or reduction.